Photon-Counting Detector CT: Advances and Clinical Applications in Cardiovascular Imaging

Abstract

Background

Since the approval of the first dual-source photon-counting detector CT (PCD-CT) in the fall of 2021, significant insights have been gained in its application for cardiovascular imaging. This review aims to provide a comprehensive overview of the current state of knowledge and the growing body of research literature, illustrating innovative applications and perspectives through case examples.

Method

We conducted a structured literature review, identifying relevant studies via Google Scholar and PubMed, using the keywords “photon-counting detector”, “cardiovascular CT”, “cardiac CT”, and “ultra-high-resolution CT”. We analyzed studies published since January 2015. Additionally, we integrated our own clinical experiences and case examples.

Results and Conclusions

In addition to the well-known benefit of increased temporal resolution offered by dual-source scanners, dual-source PCD-CT provides three key advantages: 1) Optimized geometric dose efficiency with an improved contrast-to-noise ratio, 2) intrinsic spectral sensitivity, and 3) the ability for ultrahigh-resolution CT. This technology enables improved image quality or radiation dose reduction in established cardiovascular protocols. Its use in non-invasive cardiac diagnostics for obese patients, those with a high plaque burden, or after stent implantation appears technically feasible, potentially expanding the scope of CT. The spectral sensitivity also allows tailored image acquisition, reducing metallic artifacts and contrast agent doses in patients with renal impairment. Early studies and clinical experience support these potential applications of PCD-CT in cardiovascular diagnostics, suggesting workflow optimization and improved patient management.

However, challenges remain, including high costs, large data volumes, somewhat longer reconstruction times, and technical difficulties in combining spectral sensitivity with ultra-high resolution. Prospective randomized studies with clinical endpoints are lacking to confirm the clear advantage over conventional scanners. Future research should focus on endpoint-based studies and robust cost-benefit analyses to evaluate the potential of this technology and facilitate its evidence-based integration in clinical practice.

Key Points

• Photon-counting detector CT represents a technological advancement in computed tomography.

• Spectral sensitivity enhances iodine signal and minimizes artifacts.

• Ultra-high-resolution CT allows precise imaging, even in stents and advanced sclerosis.

• This technology must be validated through endpoint-based, randomized studies.

Citation Format

• Hagar MT, Schlett CL, Oechsner T et al. Photon-Counting Detector CT: Advances and Clinical Applications in Cardiovascular Imaging. Rofo 2025; 197: 509–516

Keywords

photon-counting - cardiac - ultrasound computed tomography (US/CT) - vascular - heart - PCD-CT

Abbreviations

CHD: Coronary heart disease

CNR: Contrast-to-noise ratio

CT: Computed tomography

CTA: CT angiography

EID-CT: Energy-integrating detector CT

PCD-CT: Photon-counting detector CT

pOAD: Peripheral arterial occlusive disease

UHR-CT: Ultra-high-resolution CT

VMI: Virtual monoenergetic image reconstructions

VNCa: Virtual non-calcium (virtual calcium subtraction)

VNI: Virtual non-iodine (virtual iodine subtraction)

Introduction

Computed tomography (CT) of the heart and vascular system is an integral part of routine clinical diagnostics and is firmly anchored in international guidelines [1] [2]. According to current European registry data, an increased number of cardiac examinations using CT compared to cardiac magnetic resonance imaging was recorded for the first time in 2022 [3]. Cardiac CT, especially contrast-enhanced CT angiography (CTA) is continuously driving technological innovations and thus represents the main tool used in non-invasive cardiac diagnostics [4]. In addition, CT is playing a central role in the diagnosis of acute pathologies and the detection of chronic diseases of the cardiovascular system.

This success is due to the ongoing development of CT detector technology: after the introduction of spiral CT in the 1990s, multi-slice CT at the turn of the millennium, and dual-source technology in 2005, the first clinical approval in 2021 of photon-counting detector CT (PCD-CT) marks the latest technological innovation [5]. During the past three years of its clinical application, the advantages of PCD-CT over conventional energy-integrated detector systems (EID-CT) have been studied in detail. In addition to improved image quality and a reduction in radiation dose, PCD-CT also enables inherent spectral sensitivity in combination with high temporal resolution as a result of dual-source geometry. These properties open up new diagnostic possibilities, especially for cardiovascular imaging [6].

This review article aims to summarize the technical principles, advantages, and challenges of PCD-CT and provide a full overview of the applications of PCD-CT in cardiovascular imaging. We provide case studies in which PCD technology has been able to improve diagnostic accuracy. A detailed analysis of the technical principles has been provided already by Stein et al. in an earlier issue of this journal [7], while Hagen et al. have provided an overview of multiple clinical indications [8].

The aim of this article is to expand on the publications above and inform readers, in particular, about potential applications of PCD-CT in cardiovascular imaging.

Technical principles

In contrast to conventional EID-CT, PCD-CT is based on semiconductor technology. When a crystalline semiconductor, for example, made of cadmium telluride, is excited by an incoming X-ray photon, the energy of the photon is transferred to the electrons in the crystal lattice of the semiconductor. This energy transfer can cause an electron to be excited from the valence band to the conduction band, creating an electron-hole pair. The electron in the conduction band is accelerated towards the anode by the applied high voltage and generates a pulsating current flow. The number of electron-hole pairs generated is proportional to the energy of the absorbed X-ray photon, and the amplitude of the induced current flow correlates with the number of electron pairs. This precise detection of the X-ray photons and their energy enables more accurate imaging. In addition, no septa are required on the detector elements, which are used in EID-CT to shield optical interference, because the acceleration of the electrons on the conduction band to the anode is directional, not undirected. This results in better spatial resolution.

By applying different energy thresholds during image readout, the signal can be segmented into several energy ranges (“bins”) (e.g. below and above ~65 keV), which allows both spectral analysis and the elimination of electronic background noise by subtracting energies below a threshold of ~25 keV. This technique is particularly advantageous in contrast-enhanced examinations, because it allows equal weighting of low-energy photons, which carry the most information about soft tissue or iodine contrast, with higher-energy photons. In addition, a single anode pixel can be divided into up to four subpixel units, enabling ultra-high-resolution CT (UHR-CT) with a collimation of 120 × 0.2 mm and a spatial resolution of 110 × 110 × 160 µm. This not only enables more precise visualization of fine anatomical structures but also reduces blooming artifacts. The possibility of combining UHR-CT with ECG triggering is opening new perspectives in cardiac diagnostics. [Fig. 1] shows the schematic design of a photon-counting detector (UHR-CT) [5] [7] [9] [10]. A detailed analysis of the technical performance of the clinically approved dual-source PCD-CT was performed by Rajendran et al. [11].

Fig. 1 Schematic design of a photon-counting detector. Incoming X-ray photons are absorbed on the detector and electron-hole pairs are created. The applied high voltage of approximately 1,000 V accelerates the electrons at the pixelated anode, where a pulsatile current flow is generated whose amplitude is proportional to the energy of the incoming X-ray photon.

Technical challenges of PCD-CT

Despite the promising advantages of PCD-CT, the technology faces several major technical challenges that are particularly relevant in cardiovascular diagnostics: one main problem is charge sharing, which describes the process in which photons absorbed near the pixel boundaries share their charge between adjacent detector cells. This leads to high-energy photons being registered incorrectly as low-energy, which can impair material discrimination. Another critical factor is the K-escape effect, which occurs when incident photons knock K electrons out of the detector material. The resulting characteristic X-rays are counted as low-energy photons in the detector cells, which leads to false energy estimations. In addition, the pulse pileup effect occurs when several photons collide so closely together in large detector cells that they are registered as a single pulse with too high an energy. Smaller detector pixels can reduce this effect, but they increase the likelihood of the K-escape effect. These technical limitations are particularly important in cardiovascular diagnostics, where precise material differentiation and high-resolution imaging are essential for assessing coronary arteries and plaques.

Optimized geometric dose efficiency

In conventional EID-CT, reflective septa are required on the detector elements to shield optical interference. This can lead to a reduction of usable detector area and impact geometric dose efficiency [12]. Because PCD-CT does not require septa, it provides increased geometric dose efficiency and thus allows for potential radiation dose reduction [13]. In addition, the elimination of electronic background noise and the possibility of virtual monochromatic image reconstruction (VMIO) contribute to an improved contrast-to-noise ratio (CNR). Particularly vulnerable patient groups, such as younger adults, women, and children, benefit from the reduced radiation dose [14]. In addition, the technology allows optimization of image quality in demanding cohorts: Euler et al. showed that in CT aortography of obese patients, the generation of VMIs could improve the CNR without increasing the radiation dose [15].

New techniques in cardiac imaging

Ultra-high-resolution computed tomography

The detector units of the pixelated anode can be divided into up to four subpixel units. The separate readout of these units enables a resolution of anatomical structures of more than 20 line pairs per centimeter (lp/cm) — known as UHR-CT. Combined with ECG synchronization, both prospective sequential examinations and retrospective spiral examinations can be performed. Initial studies have shown that reconstruction particularly with a low slice thickness and a high-resolution, edge-enhancing convolution kernel enables optimal coronary diagnostics [16] and plaque characterization [17]. In an in vitro and in vivo study, UHR-CT was demonstrated to provide improved accuracy of stenosis measurements compared to reconstructions with higher slice thickness and soft convolution kernel. It is important to note that in 53% of the cases, 114 subjects had a different classification according to CAD-RADS [18]. In a prospective study of 26 high-risk patients, spectral PCD-CT demonstrated superior diagnostic accuracy compared to conventional CT in quantifying coronary artery stenosis, with a lower mean error and higher sensitivity (100% vs. 75%) and specificity (90% vs. 50%) [19]. These results could extend the indication to a higher-risk group, as coronary CT is currently recommended primarily for exclusion diagnostics in patients with low to intermediate risk [1]. Hagar et al. integrated UHR PCD-CT as a retrospective spiral examination in a protocol for CT planning before invasive aortic valve replacement [20]. Because this group has a higher prevalence of coronary heart disease (CHD) and non-invasive imaging of the coronary arteries is difficult due to existing plaques, previous stents, and contraindications to the application of glyceryl trinitrate or pharmacological rate control [21], international guidelines recommend screening using invasive diagnostic angiography [22]. The UHR-CTA showed a high diagnostic accuracy and sensitivity, so that in 54% of cases it was possible to dispense with purely diagnostic invasive catheter angiography [23]. Additionally, UHR-CT appears to open up new fields for non-invasive coronary diagnostics with known CHD. CT imaging of patients with coronary stents only receives a moderate level of recommendation for large stent diameters in the guidelines [24]. In in-vitro and in-vivo studies, UHR-CT showed very good diagnostic performance for non-invasive stent diagnostics [25] [26] [27] [28], even for stents with a diameter of less than 3 mm [29]. [Fig. 2] illustrates the potential of UHR-CT in this regard.

Fig. 2 Ultra-high resolution (UHR) CT coronary angiography of an 82-year-old patient with known coronary artery disease and stent implantation in the anterior interventricular branch (LAD). a Axial reconstruction with 0.2 mm slice thickness and vascular convolution kernel (Bv60). b Multiplanar curved reformation of the LAD. An in-stent stenosis can be ruled out. The detailed resolution of UHR CT allows differentiation of the stent lattice structure from the surrounding calcified plaques (arrow).

The potential of spectral imaging

By reconstructing VMIs at low keV values, it is possible to reduce the amount of contrast agent in coronary imaging by up to 25% without compromising image quality [30]. Initial clinical experience indicates good image quality of VMI in cardiac imaging [31]. One major advantage of PCD-CT is that it reduces image artifacts, especially blooming artifacts, which are caused by partial volume averaging and make small structures with high absorption appear larger. Thanks to the higher spatial resolution and the use of high-energy keV images, PCD-CT improves accuracy in assessing calcified plaques, which produces more precise stenosis assessment [32]. A PCD-CT-specific virtual non-iodine (VNI, “pure calcium”) algorithm allows the removal of iodine from contrast-enhanced images while preserving the calcium signal. Theoretically, this should enable quantification of coronary sclerosis using the Agatston method from contrast-enhanced images, which has been confirmed by in vitro studies. However, there is a discrepancy with the Agatston score from native CTs with false-negative findings when only minimal sclerosis is present [33]. Nevertheless, VNI showed improved performance compared to conventional virtual-native reconstruction [34] [35]. In addition, for the first time, calcium can be removed selectively from contrast-enhanced images while other materials are preserved (VNCa, virtual non-calcium). Studies have shown more precise stenosis measurement by reducing blooming artifacts using VNCa. However, inaccurate plaque subtractions can also occur [36]. The spectral sensitivity of PCD-CT allows for the creation of iodine maps, which have proven particularly useful in oncological diagnostics [37]. In cardiac CT, this can be used to quantify the extracellular volume in late-enhancement CT [38] [39] or to generate a first-pass perfusion ([Fig. 3]).

Fig. 3 Spectral CT coronary angiography of a 91-year-old patient for planning prior to aortic valve replacement. a Multiplanar reformation of the ramus circumflexus (RCx) with high-grade stenosis in mixed calcified plaque. This leads to a subendocardial to transmural perfusion deficit of the inferolateral wall in the two-chamber view of the iodine perfusion map (arrow in b). Subsequent invasive coronary angiography including intravascular ultrasound imaging confirmed the high-grade RCx stenosis. Treatment was performed by stent implantation (not shown).

Advantages of aortic imaging

ECG-triggered CT aortography is indispensable both in emergency cases, such as aortic syndromes, and in routine diagnostics, for example, for preoperative planning of cardiac surgery or to monitor chronic aortic pathologies, such as aneurysms [40]. Limitations result from the radiation exposure and the need for contrast agents that are particularly relevant for vulnerable patient groups, such as children, pregnant women, and patients with kidney disease. PCD-CT represents a promising technological advance in aortic imaging. Compared to conventional CT, it offers higher image quality when detecting endoleaks, a reduced radiation dose, and decreased need for contrast agents.

Particularly when detecting type II endoleaks occurring after endovascular aneurysm repair, PCD-CT could have advantages. These leaks, which arise from retrograde blood flow into the aneurysm sac, often via lumbar arteries or the inferior mesenteric artery, pose a challenge for imaging. In particular, small leaks with slow perfusion are difficult to identify in conventional CT scans [41]. The higher spectral resolution of PCD-CT could more clearly distinguish the contrast agent from thrombotic material, which significantly improves the sensitivity and specificity in endoleak detection. Gomollon et al. recorded an improved CNR and thus an optimized diagnostic accuracy in these studies [42]. Another advantage of PCD-CT is that it reduces the image artifacts induced by metallic implants or stents [43]. In addition, PCD-CT has significant benefits in terms of radiation exposure. In order to detect endoleaks with different dynamics and to differentiate them from calcifications, a three-phase CT protocol with native, arterial, and venous phases is often used in conventional CT, which is associated with an overall high radiation exposure. The spectral sensitivity of PCD-CT and the possibility of virtual native reconstruction make it possible to dispense with the native phase [44]. Euler et al. showed in a study of 40 patients that PCD-CT with VMI reconstructions of 40–45 keV significantly improved CNR compared to EID-CT, especially in obese patients [15]. In addition to radiation dose reduction, PCD-CT has enabled a significant reduction in the amount of contrast agent required in studies and case reports. Rau et al. reported on an 81-year-old patient with chronic renal failure who required contrast-enhanced aortoiliac CT angiography. Using PCD-CT and VMI reconstructions of 40 keV, the contrast agent dose could be reduced to 9.5 g of iodine without compromising the diagnostic image quality of the angiography [45]. This corresponded to about one-third of the usual contrast agent dose [45]. In a prospective study with 100 patients, CT angiography of the thoracoabdominal aorta using PCD-CT was compared to EID-CT at an adjusted radiation dose. It was shown that a contrast medium volume reduced by 25% with PCD-CT had a comparable image quality to EID-CT [46]. In summary, PCD-CT offers potential promise for aortic imaging. Advantages over EID scanners include higher image sharpness and contrast differentiation, as well as increased CNR as a result of VMI reconstructions. In studies, this enabled precise detection of endoleaks after endovascular therapy with lower radiation exposure and a decreased amount of contrast agent, which is particularly beneficial for patients with chronic kidney disease ([Fig. 4]). For interested readers, please refer to the detailed review of PCD-CT and aortic imaging by Zanon et al. [47].

Fig. 4 CT angiography of the aorta and iliac vessels in a 75-year-old patient. a Acquisition with a third-generation dual-source CT (axial slice, 1.0 mm slice thickness, Bv40 convolution kernel) and a standard contrast dose of 80 ml. b The same patient underwent a follow-up examination after one year on a dual-source photon-counting CT. Reconstruction using 40 keV. A constant-sized aneurysm of the left common iliac artery of 32 mm is shown (arrow). It should be noted that only 25 ml of the same contrast agent (Ultravist370, Bayer) was used.

CT angiography of peripheral arteries

CTA of the lower extremities is an important diagnostic tool for assessing vascular pathologies, especially when evaluating peripheral arterial occlusive disease (pOAD). PCD-CT offers potential based on improved CNR and reduced radiation dose compared to conventional EID-CT. In particular, the assessment of smaller arteries in the lower leg and in the periphery is limited by the limited resolution of conventional CT detectors, which sometimes requires invasive diagnostic imaging. Gruschwitz et al. conducted an in vitro study on four cadaver specimens, comparing UHR PCD-CT with the standard spectral mode (144 × 0.4 mm collimation). The UHR scans performed with a fixed CT dose-volume index of 5 mGy and a tube voltage of 120 kV showed lower image noise. A slightly reduced intraluminal CT density was measured, but a significantly improved CNR was recorded, especially when using sharp (Bv60) or ultrasharp (Bv76) convolution kernels. These results suggest that UHR PCD-CTA may also be the preferred mode for vascular diagnostics in the periphery [48]. In a cohort study, Rippel et al. compared CTA imaging using PCD-CT and EID-CT of the lower extremities. Forty patients were scanned using PCD-CT and compared with 40 characteristic-matched patients who previously underwent EID-CT. VMIs were reconstructed from PCD-CT data at 40–120 keV, while low-kVp protocols (80 and 100 kVp) were used for EID-CT. It was shown that 40–60 keV VMIs of PCD-CT had significantly higher CNR values compared to 80 kVp and 100 kVp EID-CT. Subjective image quality ratings for vessel sharpness, attenuation and overall quality were higher for PCD-CT at lower keV values, but there were no significant differences at higher keV values [49]. A study to optimize reconstruction parameters for PCD-CT angiography of the lower extremities showed that the Qr60 core combined with the highest QIR level (QIR-4) provided the best image quality. This led to a significant reduction in noise while maintaining image sharpness, which was a particular advantage when assessing plaques and vessel walls [50]. Augustin et al. evaluated the image quality and diagnostic accuracy of PCD-CTA for the diagnosis of pOAD in 39 patients (78% with advanced pOAD). Different vascular convolution nuclei (Bv36, Bv48, Bv56) were reconstructed and compared with digital subtraction angiography (DSA) as a reference standard. The sensitivity for stenosis quantification remained stable (~81%) regardless of the type of convolution kernel, but specificity improved when using sharper kernels, with the Bv56 kernel showing the highest diagnostic agreement with DSA [51]. [Fig. 5] shows a patient for pOAD diagnosis on PCD-CT and illustrates the importance of using sharp convolution kernels. It is expected that PCD-CT will be able to replace purely diagnostic invasive angiography in the future.

Fig. 5 Axial CT angiography of the lower extremity of a 69-year-old patient with known peripheral arterial occlusive disease (pOAD). a Reconstruction in a soft kernel (Bv40) and large field of view (FOV). b Reconstruction in a small FOV restricted to the respective extremity and a hard convolution kernel (Bv60). Even in very small arteries, calcified plaque can be differentiated from the lumen, allowing a more accurate assessment of stenosis (arrow).

Outlook

PCD-CT represents the next technological development stage in CT imaging. The design of the approved detector, which combines the high temporal resolution of dual-source technology with a photon-counting semiconductor detector plate technology, has proven to be particularly promising in cardiovascular imaging. There is broad consensus on the fundamental benefits of PCD technology, which is supported by numerous scientific publications. In the coming years, PCD-CT could gradually replace EID-CT. However, many questions remain unanswered: On the one hand, a combination of spectral sensitivity and UHR-CT is currently not possible without technical compromises, which still limits the full potential of this technology, since not all theoretical advantages of PCD-CT are available in every acquisition. Initial exploratory studies suggest an improved clinical benefit of PCD-CT, but prospective randomized, endpoint-based studies are still lacking. Nevertheless, these are necessary before specific adjustments to the PCD-CT guidelines can be made and clinical processes can be optimized sustainably. Furthermore, the cost-benefit efficiency has not yet been investigated sufficiently.

Summary

PCD-CT represents a significant advance in CT imaging, particularly in cardiovascular diagnostics. By improving image quality, reducing radiation exposure, and increasing diagnostic accuracy, some studies have raised hopes that PCD-CT could overcome some of the limitations of conventional CT systems. Despite the proven benefits, wide-ranging prospective studies are needed to further validate its clinical use. This review article provides a full overview of the current state of research, and it examines the principles and potential applications for PCD-CT in cardiovascular diagnostics.

Conflict of Interest

MTH Speakers’ bureau (Siemens Healthineers) CLS Speakers’ bureau (Siemens Healthineers) AVS Unrestricted research grant, speakers’ bureau (Siemens Healthineers) TE Unrestricted research grant, speakers’ bureau (Siemens Healthineers) XC Is an employee of Siemens Healthineers FB Unrestricted research grant, speakers’ bureau (Siemens Healthineers, Bayer Healthcare) All other authors declare no potential conflicts of interest.

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Publication History

Received: 08 September 2024

Accepted after revision: 17 October 2024

Article published online:
20 November 2024